1. Field of the Invention
The present invention relates to devices for detecting scour around bridge foundations. More particularly, the present invention relates to dynamic devices for monitoring the condition of the river bed adjacent to bridge foundations and piers.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Total scour in a river channel occurs as a combination of three phenomena: General scour that may be influenced by contractions of the channel in the vicinity of a bridge, natural degradation of the channel due to active geologic processes, and local scour caused by local flow disturbances (primarily vortices) around piers and abutments. Most scour is at least in part a combination of two or more of these components, and it is difficult to separate the effects of each. Scour can occur in almost all soils, but the rate of scour is highest in cohesionless sands, gravels and silts; more moderate in dispersive clays; and slowest in non-dispersive clays and cemented granular soils. Given a flood, or series of floods, of sufficiently long duration, most soils will tend to scour to about the same depth. Scour depths of up to 60 feet have been inferred from the Salt River floods, and the reconstructed foundations have been designed with deep foundations to resist scour to these depths. Most of the replacement foundations have been deep, cylindrical drilled shafts, which are more streamlined than the driven pile sections that were originally used to support the bridges, since the circular shape of the drilled shaft will reduce the depth of local scour.
The scour in the immediate proximity of a bridge foundation is generally of more concern to the foundation engineer, however, than scour occurring far from piers or abutments. For example, for a footing bearing on the surface of cohesive soil with no frictional component of shear strength, the scouring of sediments producing a dip of 60 degrees in the surface of the sediments within a distance to the foundation equal to 75 percent of its width has no effect on ultimate bearing capacity, while the development of a scour zone with the same dip angle at and parallel to the edge of the footing reduces its bearing capacity by 30 percent. Such partial undercutting may not render the footing unsafe with respect to ultimate failure, but it may produce settlement, endangering the bridge's serviceability. A strong probability exists that scour-induced movements, which may be combinations of vertical and horizontal movements, in excess of 1.0 inch will not be tolerable by typical highway bridges. The settlement characteristics of the foundation will depend on the shape of the scour zone and the rate at which infilling (if any) occurs, and the type of infilled soil that may be deposited. It appears logical, therefore, that studies of scour that focus on the undermining of bridge foundations focus largely on the combined effects of general scour, degradation and local scour near foundations.
The effect of scour around bridge foundations has long been a consideration in bridge design, both from the point of view of the river hydraulics specialist and from that of the foundation engineer. Numerous bridge failures have been attributed to the undermining of interior piers or abutments by scour during floods. If the depth of scour in the vicinity of the pier or abutment exceeds the design limits, excessive movements of the foundation can occur, perhaps exceeding the service limit of the structure, even to the extent of completely undermining the foundation and producing collapse of the structure. Collapse from scour unfortunately often results in the loss of life but always causes serious traffic disruptions and expensive repairs. The Federal Government spends over $50,000,000 annually on the nation's primary road system for emergency bridge repairs, primarily from flood induced scour. Local and state agencies spend over twice that amount annually on emergency repairs to scour-damaged bridges.
A recent publication of the Federal Highway Administration cites the following recent statistics regarding the loss of bridges due to scour: In the spring floods of 1987, 17 bridges in New York and New England were destroyed or damaged by scour, and in 1985 73 bridges were destroyed by floods in West Virginia, Virginia and Pennsylvania, with damage being distributed approximately equally between piers and abutments. Severe floods in the western U.S. have also produced scour that have destroyed or made unserviceable major highway bridges, an example of which were recent floods on the Salt River in Arizona, which completely undermined pile-supported piers.
A major problem facing highway engineers is the lack of a simple, inexpensive scour detection device that can withstand the forces of floods and the impact of flood-borne debris. It is very important that a successful scour-detection device be able to provide real-time information to the highway engineers during a flood to warn of dangerous scour conditions.
Considerable serious bridge scour research has been conducted, but most of this research has been confined to the laboratory. This research has resulted in the development of methods, both semi-analytical and empirical, to predict scour depth, and these equations, among others, are currently used by bridge designers to predict scour depth for foundation design. Most of these methods were developed to apply to a particular type of scour (e.g., local scour at abutments), for particular ranges of Froude numbers in the scouring stream, and for particular foundation configurations and soil characteristics. Predicted scour depths vary according to these procedures, especially when applied outside of the range of parameters for which they were developed.
Some states, such as Minnesota, have begun systematic programs to catalog scour susceptible bridges and to outline rational procedures for further in-depth study, and in 1988, the Federal Highway Administration issued a technical advisory recommended the development and implementation of scour evaluation programs in all states. The acquisition of field data on scour, to augment data acquired from laboratory modeling, has been recognized as being essential to the improvement of the understanding and predictive capabilities regarding the estimation of the depth of scour, configurations of scour zones and the nature of re-deposited soils in the scour zone, if any. Such field studies require appropriate and reliable instrumentation.
Presently, there are a number of instruments that can be used to measure scour. The simplest and cheapest of all instruments are graduated poles and weighted lines that can be lowered to the river bed during and after a flood event from a bridge or from a boat. These devices are impractical in deep water and during high-velocity flow. Heat dissipation gages are fixed gages that consist of an assembly of electrical heaters and temperature sensors that are fastened together in a long rod, which is driven or jetted into the stream bed near a bridge pier. The wires leading from the heat dissipation gage assembly are routed to a central point on the bridge. During the flood event, an operator connects a battery to the sensors. The sensors above the river bed cool much faster than the sensors below the bed, such that the depth of scour at the location of the assembly is determined directly by comparing temperature records at various times. Resistance gages are used which include a metal sensing rod which is fixed directly on a pier or abutment with wires leading to a central point on the bridge. During a flood event, an operator connects a battery to the sensing rod to warm it. After a period of time, the battery is disconnected and the rod is allowed to cool. As the rod cools, it contracts much faster above the stream bed than below, and the strain gages on the rod can detect this differential contraction, allowing the operator to assess the depth of scour.
A fathometer can also be used so as to measure scour conditions. Fathometers are instruments which send and detect acoustic pulses electronically that are reflected from the river bottom and shallow sediment interfaces and accurately measure the depth of the scoured soil surface. Color fathometers are also available that process the reflected signal and allow for the interpretation of the depth and character of infilled soils. Fathometers are portable and can be used readily to investigate general scour patterns after a flood event, even in high velocity streams; however, they are problematical to use during a flood, since a boat carrying the fathometer must move from location to location.
Ground-penetrating radar can be used so as to penetrate the water column and soil strata immediately below the stream bed so as to provide a profile of the soils below the stream bed. The length of the electromagnetic radar waves at the frequency is about 0.4 meters at 80 MHz in water so as to give a resolution of about one foot. Higher frequencies can be used to improve resolution but with a severe penalty in sediment penetration. Since it is desirable to collect data on the presence and nature of infilled soils, high penetration is more important than high resolution. Ground-penetrating radar suffers from the disadvantage that it cannot be operated effectively and safely during a severe flood, except from the bridge. A technical disadvantage of this type of instrumentation is that the presence of a clay load or salt water in the stream severely attenuates the signal and produces poor resolution. As such, penetration of the water surface of more than about 25 feet would not be practically possible.
Side-scan sonar can be used so as to measure scour conditions by providing continuous acoustic pulses which are emitted at high frequency such that reflections are sensed by an acoustic transducer tuned to specific frequencies. Commercial side-scan sonar devices have rotating heads that allow for profiling of the soil surface away from the location of the instrument. It is usually operated by towing from a boat.
Seismic surveys are another technique of measuring scour conditions. Seismic surveys can be carried out in accordance with prior patent to the present inventor (U.S. Pat. No. 5,753,818). This seismic technique is based upon installing an instrument access tube adjacent to and parallel to the bridge pier. A vertical array of hydrophones are inserted into this tube and record refracted events generated by a hammer blow on the pier which propagates down the pier and the water over the mud/soft sand over competent soil interface. The seismic energy is analyzed for deviations in first-break time to detect the low velocity mud/soft-sand zone/competent soil interface at the bottom of the scour zone. This seismic method requires installation of an access tube to hold the hydrophone array. It is possible that flood event debris could damage the tube and render it ineffective during the flood event.
It is an object of the present invention to provide an apparatus which measures scour depth profile dynamically.
It is another object of the present invention to provide a scour detection instrument which provides accurate sediment profile resolution in all types of sediments and waters.
It is another object of the present invention to provide a scour detection instrument which is portable.
It is another object of the present invention to provide a scour detection instrument which is of low cost.
It is a further object of the present invention to provide a scour detection instrument which is very reliable and which can perform under extreme flood conditions.
It is still another object of the present invention to provide a scour detection instrument which is easy to operate.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.